Various types of digitizers are known wherein a cursor is moved over a surface beneath which there is a grid with one of the cursor or grid being energized and the other being influenced by the energization to provide a signal indicative of the cursor location with respect to the grid. It is desirable to be able to measure the distance of a point from orthogonal X and Y axes on a surface in order to determine the coordinates of the point by placing a stylus or cursor at the point without concern for keeping the cursor in engagement with the surface as it is moved from the origin at the intersection of the axes to the point. Various devices are known which accomplish this objective and some are discussed in U.S. Pat. No. 4,054,746 to Kamm for Electronic Coordinate Position Digitizing System.
In order to achieve high resolution for accurate determination of coordinates it is desirable to limit the distance range over which a fine measurement is made and to re ference the fine determination to a secondary axis which is a known distance from the parallel origin axis. As is disclosed in the above-referenced Kamm patent, a coarse grid can be used in conjunction with a fine grid and a common cursor. The coarse grid and cursor can be used to determine the element of the fine grid closest to the cursor and the fine grid and cursor can serve to determine the distance of the cursor from the closest coarse grid element with the closest coarse grid element serving as an auxiliary axis a known distance from the parallel origin axis. In one apparatus in the prior art, a coarse grid is provided adjacent a fine grid with the parallel coarse grid elements being equally spaced at one half the spacing of the fine grid elements, adjacent elements defining grid cycles, so that there are two coarse grid cycles in registration with each fine grid cycle. The coarse grid elements are sequentially energized, one at a time, and the signal induced in the inductively coupled cursor by each coarse grid element as it is energized is applied to a threshold circuit. The threshold circuit is set so that only the coarse grid element closest to the cursor will be able to provide a signal sufficient to cause an output at the threshold circuit which identifies that grid element as the one closest to the cursor. In order to resolve ambiguities it is necessary that there be two coarse grid cycles for every one fine grid cycle.
In addition to the necessity for including twice as many coarse grid cycles as there are fine grid cycles in the prior art, such coarse measuring devices suffer from another disadvantage in that they are amplitude dependent and vulnerable to the problem of amplitude dependent systems which results in erroneous data readings when the cursor is lifted from the coordinate surface or when there are line voltage fluctuations.